如何使用 Flutter 下载文件并将其存储在 Downloads 文件夹中

Question

我正在使用 Flutter 1.10.3 构建应用程序，但无法将图像下载到设备的下载文件夹。

有没有不使用任何第三部分库的选项？

Answer 1

添加一些必需的依赖项，我只有 android 的测试演示。请查看file_utils并path_provider在您在 IOS 设备中实施时。

  dio: ^3.0.0

  path_provider: ^1.3.0

  simple_permissions: ^0.1.9

  file_utils: ^0.1.3

您需要在 android mainfest 文件上添加权限。

  <uses-permission android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

示例代码：

  import 'dart:io';
  import 'package:flutter/material.dart';
  import 'package:dio/dio.dart';
  import 'package:path_provider/path_provider.dart';
  import 'dart:async';
  import 'package:simple_permissions/simple_permissions.dart';
  import 'package:file_utils/file_utils.dart';
  import 'dart:math';

  void main() => runApp(Downloader());

  class Downloader extends StatelessWidget {
    @override
    Widget build(BuildContext context) => MaterialApp(
          title: "File Downloader",
          debugShowCheckedModeBanner: false,
          home: FileDownloader(),
          theme: ThemeData(primarySwatch: Colors.blue),
        );
  }

  class FileDownloader extends StatefulWidget {
    @override
    _FileDownloaderState createState() => _FileDownloaderState();
  }

  class _FileDownloaderState extends State<FileDownloader> {

    final imgUrl = "https://images6.alphacoders.com/683/thumb-1920-683023.jpg";
    bool downloading = false;
    var progress = "";
    var path = "No Data";
    var platformVersion = "Unknown";
    Permission permission1 = Permission.WriteExternalStorage;
    var _onPressed;
    static final Random random = Random();
    Directory externalDir;

    @override
    void initState() {
      super.initState();
      downloadFile();
    }


    Future<void> downloadFile() async {
      Dio dio = Dio();
      bool checkPermission1 =
          await SimplePermissions.checkPermission(permission1);
      // print(checkPermission1);
      if (checkPermission1 == false) {
        await SimplePermissions.requestPermission(permission1);
        checkPermission1 = await SimplePermissions.checkPermission(permission1);
      }
      if (checkPermission1 == true) {
        String dirloc = "";
        if (Platform.isAndroid) {
          dirloc = "/sdcard/download/";
        } else {
          dirloc = (await getApplicationDocumentsDirectory()).path;
        }

        var randid = random.nextInt(10000);

        try {
          FileUtils.mkdir([dirloc]);
          await dio.download(imgUrl, dirloc + randid.toString() + ".jpg",
              onReceiveProgress: (receivedBytes, totalBytes) {
            setState(() {
              downloading = true;
              progress =
                  ((receivedBytes / totalBytes) * 100).toStringAsFixed(0) + "%";
            });
          });
        } catch (e) {
          print(e);
        }

        setState(() {
          downloading = false;
          progress = "Download Completed.";
          path = dirloc + randid.toString() + ".jpg";
        });
      } else {
        setState(() {
          progress = "Permission Denied!";
          _onPressed = () {
            downloadFile();
          };
        });
      }
    }

    @override
    Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text('File Downloader'),
        ),
        body: Center(
            child: downloading
                ? Container(
                    height: 120.0,
                    width: 200.0,
                    child: Card(
                      color: Colors.black,
                      child: Column(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: <Widget>[
                          CircularProgressIndicator(),
                          SizedBox(
                            height: 10.0,
                          ),
                          Text(
                            'Downloading File: $progress',
                            style: TextStyle(color: Colors.white),
                          ),
                        ],
                      ),
                    ),
                  )
                : Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Text(path),
                      MaterialButton(
                        child: Text('Request Permission Again.'),
                        onPressed: _onPressed,
                        disabledColor: Colors.blueGrey,
                        color: Colors.pink,
                        textColor: Colors.white,
                        height: 40.0,
                        minWidth: 100.0,
                      ),
                    ],
                  )));
  }

Answer 2

如果您控制服务器，另一种选择是让用户的系统处理下载。在服务器上，您Content-Disposition: attachment向响应添加标头。在 nginx 中，您可以使用add_header启用站点的配置文件中的指令执行此操作：

add_header Content-Disposition "attachment";

更多关于这里的说明。

然后在 Flutter 端你可以使用url_launcher包来启动下载：

await launch('https://www.example.com/my_file.pdf');

这会将下载交给手机上安装的浏览器。而Content-Disposition: attachment头将告诉浏览器，这是被下载，而不是显示或播放。

此解决方案非常轻松，但它无法让您控制文件下载到的位置。